A method of manufacturing a non-planar HBT transistor is already known from U.S. Pat. No. 4,731,340. According to this known method, lift-off techniques are used to obtain self-alignment of base contacts on the emitter. A photoresist mask defining an emitter adjacent to the base contact is formed on a support suitable to constitute a bipolar heterojunction of semiconductor material. A base contact is formed by etching through the upper semiconductor layer as far as the heterojunction and a metal is deposited on the lower semiconductor layer of the heterojunction. A dielectric is then deposited on the base contacts formed beforehand and then the photoresist mask is eliminated, which constitutes the technique of lift-off of the dielectric and of the metal in the regions in which they are undesirable. At this instant, the emitter contact can be formed due to the fact that the base contact is still covered by the dielectric. The method is terminated by the formation of an opening as far as the deepest layer and the formation of the collector contact on the layer.
This method offers the advantage of obtaining self-alignment of the base on the emitter region and then forming the emitter contact by a method which is not very critical. Moreover, the edges of the emitter are protected by a dielectric.
However, this method has the disadvantage of not forming the collector contacts sufficiently close to the base contacts, as a result of which the parasitic intrinsic elements associated with the transistor and especially the base-collector capacitances are increased.
The parasitic elements considerably reduce the speed of the HBT transistors. On the other hand, it is important to reduce the dimensions of the HBT transistors in order to reduce their consumption, which also influences the speed.